CHEMISTRY 253

Spring, 2015 - Dixon

Homework Set 1.5 Solutions (Non-collected Problems)

Review Questions

4-1  Discuss the relationship between atmospheric particulates and haze. 

Atmospheric particulates, particularly in the fine mode, are most responsible for the loss of visibility or visible light extinction through light scattering.

4-2  What is acid rain?  What two acids predominate in it?  Explain why the predominant acid in acid rain differs in eastern and western North America.

Acid rain is rain made more acidic that naturally expected in the absence of anthropogenic emissions.  The two most significant acids are nitric and sulfuric acids.  There is a difference in the predominant acid between eastern and western North America because in the east, coal is used as a fuel source for power and typically contains moderate to high levels of sulfur.  In western North America, coal is a less significant power source, and typically contains low levels of sulfur.

4-3  What is the difference between dry and wet deposition?

Dry deposition is the term use for the removal of compounds in dry weather through processes, besides precipitation, by which atmospheric constituents get removed to the ground.  This can consist of particle sedimentation, or absorption of sulfur dioxide onto wet surfaces.  Wet deposition refers to precipitation-based processes.

4-4  Using chemical equations, explain how acid rain is neutralized by limestone that is present in soil.  Describe ways in which humans have tried to neutralize acidified lakes.

Limestone is primarily CaCO₃ and reacts with acid as follows:

                        2H⁺ + CaCO₃ (s) ↔ Ca²⁺ + H₂CO₃ (aq) ↔ CO₂ (g) + H₂O (l)

Lime can also be added to acidified lakes to cause the above reaction.  Another way to acidify lakes is to add phosphate to generate plant growth.  This leads to denitrification which also consumes acid.

4-6  Describe the effects of acid precipitation upon a) dissolved levels of aluminum, b) fish populations, and c) trees.

Acid precipitation makes aluminum more soluble (since it can react with hydroxides released by the dissolution of Al(OH)₃, leading to increases in its concentration.  The dissolution of aluminum and high levels of acidity are not tolerated well by many fish.  These levels can result in fish die-offs or in decreased reproduction. Acidic precipitation can affect trees through dissolution of aluminum, although acidic fogs are also thought to be unhealthy for trees.

4-7  Explain the differences between soot-and-sulfur-dioxide smog and photochemical smog in terms of the chemicals involved and the health effects they cause.

Soot-and-sulfur-dioxide smog tends to be high in particulate levels and caused by emissions of sulfur dioxide and combustion aerosols.  It does not need sunlight, but does need oxidation of sulfur dioxide.  The main health effect comes from the efficiency by which fine aerosol particles can make it into lungs.  Photochemical smog results in the production of ozone and oxidized organic molecules.  These are also harmful to lung functioning.